High gain antennas are widely useful for communication purposes such as radar, television receive-only (TVRO) earth station terminals, and other conventional sensing/transmitting uses. In general, high antenna gain is associated with high directivity, which in turn arises from a large radiating aperture.
U.S. Pat. No. 4,845,507 discloses a modular radio frequency array antenna system including an array antenna and a pair of steering electromagnetic lenses. The antenna system of this patent utilizes a large array of antenna elements (of a single polarity) implemented as a plurality of subarrays driven with a plurality of lenses so as to maintain the overall size of the system small while increasing the overall gain of the system. Unfortunately, the array antenna system of this patent cannot simultaneously receive both right-hand and left-handed circularly polarized signals, and furthermore cannot simultaneously receive signals from different satellites wherein the signals are right-handed circularly polarized, left-handed circularly polarized, linearly polarized, or any combination thereof.
U.S. Pat. No. 5,061,943 discloses a planar array antenna assembly for reception of linear signals. Unfortunately, the array of this patent, while being able to receive signals in the fixed satellite service (FSS) and the broadcast satellite service (BSS) at 10.75 to 11.7 GHz and 12.5 to 12.75 GHz, respectively, cannot receive signals (without significant power loss and loss of polarization isolation) in the direct broadcast (DBS) band, as the DBS band is circular (as opposed to linear) in polarization.
U.S. Pat. No. 4,680,591 discloses an array antenna including an array of helices adapted to receive signals of a single circular polarization (i.e. either right-handed or left-handed). Unfortunately, because satellites transmit in both right and left-handed circular polarizations to facilitate isolation between channels and provide efficient bandwidth utilization, the array antenna system of this patent is blind to one of the right-handed or left-handed polarizations because all elements of the array are wound in a uniform manner (i.e. the same direction).
It is apparent from the above that there exists a need in the art for a multiple beam array antenna system (e.g. of the TVRO type) which is small in size, cost effective, and modular so as to increase gain without significantly increasing cost. There also exists a need for such a multiple beam array antenna system having the ability to receive each of right-handed circularly polarized signals, left-handed circularly polarized signals, and linearly polarized signals. Additionally, the need exists for such an antenna system having the potential to simultaneously receive signals from different satellites, the different signals received being of the right-handed circularly polarized type, left-handed circularly polarized type, linearly polarized typed, or combinations thereof. It is the purpose of this invention to fulfill the above-described needs in the art, as well as other needs apparent to the skilled artisan from the following detailed description of this invention.
Those skilled in the art will appreciate the fact that array antennas are reciprocal transducers which exhibit similar properties in both transmission and reception modes. For example, the antenna patterns for both transmission and reception are identical and exhibit approximately the same gain. For convenience of explanation, descriptions are often made in terms of either transmission or reception of signals, with the other operation being understood. Thus, it is to be understood that the array antennas of the different embodiments of this invention to be described below may pertain to either a transmission or reception mode of operation. Those skilled in the art will also appreciate the fact that the frequencies received/transmitted may be varied up or down in accordance with the intended application of the system. Those of skill in the art will further realize that right and left-handed circular polarization may be achieved via properly summing horizontal and vertical linearly polarized elements. It is also noted that the array antenna to be described below may simultaneously receive and transmit different signals.